This application relates to low-voltage incandescent lamps of the type with tungsten filaments and, in particular, to lamps of the type having voltage-conversion circuitry built into the lamp.
It is known to provide incandescent lamps which operate at a voltage below the standard AC line voltage, typically 120 VAC. For example, lamps operating at 12 volts are known for a variety of applications. In some instances, a specialized fixture is provided for such lamps, the fixture including conversion circuitry for converting the AC line voltage to the operating voltage of the lamp.
It is also known to provide low-voltage lamps which are designed to be powered from standard AC line sockets or fixtures. In such lamps, the voltage conversion circuitry is incorporated into the lamp itself. Such a lamp is disclosed, for example, in U.S. Pat. No. 4,998,004, in which the conversion circuitry is disposed in the base and inside the envelope of the lamp. The lamp disclosed in that patent is a 40-watt incandescent bulb designed to operate at 15 volts. The wattage of the lamp is necessarily low because, at higher wattages, the temperature generated in the lamp may be damaging to the voltage conversion circuitry. In converting the line voltage to a lower voltage, the lamp current is increased, generating greater heat from the filament. In a halogen lamp, this high temperature is necessary in order to keep the halogen in cycle. The heat is conducted by the filament leads, is convected from the wall of the halogen capsule and is radiated from the filament, so that the temperature inside the envelope and the base builds up to a substantial level. For example, it has been found that for a 50-watt halogen lamp with reflector and lens, operated in a base-up configuration inside a recessed can fixture, the temperature inside the lamp base will reach in excess of 120xc2x0 C. Many electronic components do not operate well at such temperatures. Additionally, the voltage conversion circuitry used in the lamp of U.S. Pat. No. 4,998,044 can itself generate additional heat.
In U.S. Pat. No. 6,147,457 there is disclosed a low-voltage incandescent lamp with an inverter driven by a control circuit which generates less heat than the conversion circuitry of the lamp of U.S. Pat. No. 4,998,044, but it does not solve the temperature problem, since most of the heat generated by a low-voltage lamp is generated by the filament.
The high temperatures which can be experienced in a low-voltage lamp affect not only semiconductor performance, but also the lifetime of electrolytic capacitors. If the lamp is one, such as a parabolic aluminized reflector (xe2x80x9cPARxe2x80x9d) halogen lamp, with a replaceable halogen capsule, the useful life of the conversion circuitry should be several times the life of the replaceable capsule and such lifetimes are not possible at the high temperatures experienced in low-voltage lamps, particularly when operated in a base-up configuration, except at very low wattages.
When the lamp is operated in a base down or horizontal configuration, heat buildup is less of a problem, so that high-heat-intolerant circuit components could be incorporated in the base of the lamp, except for the fact that the normal base does not typically have adequate room to accommodate the entire conversion circuitry.
This application discloses an improved low-voltage incandescent lamp which avoids the disadvantages of prior lamps while affording additional structural and operating advantages.
An important aspect is the provision of a low-voltage incandescent lamp with a built-in voltage conversion circuit which can withstand high-temperature applications.
In connection with the foregoing aspects, a further aspect is the provision of a lamp of the type set forth which mounts high-temperature-intolerant portions of the conversion circuit at a location remote from the base.
In connection with the foregoing aspects, a further aspect is the provision of a lamp of the type set forth, wherein the high-temperature-intolerant portion of the conversion circuit is disposed externally of the lamp.
A still further aspect is the provision of the lamp of the type set forth which may be provided in reflectorized or non-reflectorized form and is mountable in a standard 120 VAC socket.
Certain ones of these and other aspects may be attained by providing an incandescent lamp adapted to be powered from a fixture connected to an ordinary source of AC line voltage, comprising: a base for connection into an associated fixture, an envelope having a light-transmitting portion connected to the base, a voltage conversion circuit for converting the AC line voltage to a lower output voltage, the conversion circuit having a high-temperature-tolerant portion disposed in the base and a high-temperature-intolerant portion disposed outside the envelope and the base, and a light-generating filament disposed in the envelope and adapted to be coupled to the output voltage for being powered thereby.